A. Field of the Invention
This invention relates to methods for synthesizing MCM-48. The processes of the invention include reacting an inorganic silica reagent, an alkylammonium hydroxide, and a halide-containing surfactant in an aqueous medium. The process can be carried out in a single step or in multiple steps such that the inorganic silica reagent and the alkylammonium hydroxide are reacted first to form a first reaction mixture. This first reaction mixture then is combined with a halide-containing surfactant to form a second reaction mixture that is maintained under sufficient conditions to form a crystalline MCM-48 product. Preferably, the MCM-48 synthesis according to this invention is accomplished using the same convenient reagents and processing as for MCM-41, i.e., in an aqueous medium that is alcohol free, using readily available commercial reactants that are relatively safe and easy to handle.
B. Description of the Prior Art
MCM-48 is a crystalline, high silica-containing molecular sieve material and is described, for example, in U.S. Pat. No. 5,198,203, which is hereby incorporated by reference. It is a member of a family of mesoporous materials known by the designation "M41 S." In addition to MCM-48, other members of the M41S family of materials include MCM-41 and MCM-50.
MCM-48 differs from MCM-41 and MCM-50 in its crystal and pore structures. MCM-41 has a hexagonal crystal structure with a uni-dimensional pore system, while MCM-50 has a lamellar structure. MCM-48, on the other hand, has a cubic Ia3d symmetrical structure, with a proposed three-dimensional pore system, like that shown schematically in FIG. 1, having two independent intertwined channel networks. Because of this three-dimensional pore structure, MCM-48 is an attractive candidate for use in various sorption and catalysis applications (e.g., this three-dimensional pore structure provides high surface area for adsorption and contacting reactants).
While attractive because of its possible three-dimensional pore structure, crystalline MCM-48 has not been widely used. Difficulties in synthesizing MCM-48, in the laboratory and particularly on a large scale commercial basis, have limited the availability of this material. While various methods for synthesizing MCM-48 are known, each has significant drawbacks. For example, much of the MCM-48 produced by known methods has been produced using alkoxide-based organic silica sources, such as tetraethylorthosilicate (Si(OC.sub.2 H.sub.5).sub.4 or "TEOS") or its homologues. See, for example, Q. Huo et al., "Surfactant Control of Phases in the Synthesis of Mesoporous Silica-Based Materials," Chemistry of Materials, Vol. 8 (1996), pp. 1147-1160. These reagents, however, present significant handling problems (e.g., high toxicity, moisture sensitivity) and are quite costly, making large-scale synthesis of crystalline MCM-48 by this procedure impractical.
Another known procedure for making MCM-48 uses conventional silica sources, but this procedure also requires addition of alcohol during the synthesis. See, for example, U.S. Pat. No. 5,300,277. The MCM-48 product produced by the procedure described in this patent had lower quality (as determined by X-ray diffraction) than that prepared from TEOS. Additionally, this synthesis process generated high pressures when the temperature was elevated during the synthesis.
Corma et al., "Synthesis of Si and Ti--Si--MCM-48 Mesoporous Materials with Controlled Pore Sizes in the Absence of Polar Organic Additives and Alkali Metal Ions," Journal of the Chemical Society, Chemical Communications, (1998), pp. 579-580, describe an alcohol-free MCM-48 synthesis. The surfactant solution used in this synthesis process is an aqueous solution of cetyltrimethylammonium hydroxide/bromide (C.sub.16 TMAOH/Br) with an OH/Br ratio of 90/10 (i.e., 90% C.sub.16 TMAOH and 10% C.sub.16 TMABr). This hydroxide-based surfactant material, however, is difficult to obtain and relatively expensive. The typical method of obtaining such hydroxide-based surfactants is by substitution of the halide ion in a halide-based surfactant, e.g., by ion exchange with hydroxide. This approach to generating a C.sub.16 TMAOH/halide (Cl) mixture was disclosed in U.S. Pat. No. 5,102,643 in the context of the M41S family in general. Thus, the MCM-48 synthesis described by Corma is likely preceded by a substitution step (OH for Br), which could be avoided if the commercially available halide-containing surfactant solution could be used directly, as in the present case.
Thus, advancement and testing of crystalline MCM-48 products have been hampered by the unavailability of this material due to the difficult, expensive, elusive, and not consistently reproducible synthesis procedures required to produce it. There is a need in the art for a convenient method for synthesizing high silica MCM-48. Preferably, this method would use the same simple combination of reagents than can be used to produce the other members of the M41S family, i.e., aqueous, alcohol-free media and reagents that are commercially available and relatively easy to handle, e.g., precipitated silica, surfactant chloride, and tetramethylammonium hydroxide. Additionally, this method preferably will be capable of large-scale production of MCM-48 in a commercially viable manner.